A marked decrease in behaviorally measured threshold for electrical stimulation, applied through permanently implanted electrodes, is referred to as stimulation hypersensitivity. In previous work with cat, stimulation hypersensitivity in several locations throughout the brain stem auditory system was obtained following mechanical damage of the cochleas. The proposed work will center on cochlear nucleus and will determine if stimulation hypersensitivity can be produced by sound-induced hearing loss. Related objectives are: to determine whether hypersensitivity in cochlear nucleus can be produced by acoustic trauma to limited regions of the cochlea; to monitor the time course over which hypersensitivity develops; and to relate stimulation-threshold changes with subdivision of cochlear nucleus (anteroventral, posteroventral, dorsal) and with cell type in the vicinity of the stimulating electrode. Stimulation hypersensitivity is a new topic of investigation in the field of hearing that is relevant to the understanding of auditory functioning in the behaving animal and in human.